1. Field of the Invention
This invention relates to a method of extracting outline data from bi-level image data. This invention also relates to an apparatus for extracting outline data from bi-level image data.
2. Description of the Prior Art
Image scanners convert printed characters on a sheet into corresponding image information. There are various ways of extracting data representative of outlines of the characters from the image information.
Some printers and computers have ROMs storing data representative of character dot patterns. For enlarging and reducing such character patterns, it is known to extract outline data from character data and to convert the outline data into outline font data.
Japanese published unexamined patent applications 62-131382 and 64-91176 disclose similar methods of extracting outline data from bi-level (black-white) image data. In the methods of Japanese applications 62-131382 and 64-91176, input bi-level image data is expanded twice in each of vertical and horizontal directions, and then a decision is made as to whether data of pixels are black ("1") or white ("0"). This decision is to find a point (pixel) from which tracking and tracing an outline of a black region is started. After such a starting pixel is found, 8 pixels neighboring the starting pixel are checked to detect a direction along which a 0-to-1 change in pixel data is present. Movement along the detected direction corresponds to tracing a fragment of the outline. Successive execution of such movements will complete tracing the entire outline. Specifically, the methods of Japanese applications 62-131382 and 64-91176 enable tracing the outline which connects the centers of pixels extending along the boundary between a black region and a white region.
The methods of Japanese applications 62-131382 and 64-91176 have the following problems. Since input bi-level image data is expanded twice in each of vertical and horizontal directions and then an expanded (larger) quantity of image data is processed, a large-capacity memory is necessary and a long time is spent in processing image data. Since the outline connecting the centers of pixels extending along the boundary between a black region and a white region is traced, a positional error tends to occur between the outline in an original image and the outline in a reproduced image resulting from the processing of the original image. Since a check to detect a direction related to a 0-to-1 change is periodically executed many times, a large number of processing steps is necessary and a long time is spent in the data processing.
Japanese published unexamined patent application 64-68889 discloses a method of detecting right-angled points in an outline of a black region. Specifically, the method of Japanese application 64-68889 uses a sequence of chain code words which represents the outline connecting the centers of pixels extending along the boundary between a black region and a white region. Analyzing the sequence of chain code words decides bending conditions of the outline of the black region so that right-angles points in the outline can be detected. Pixels neighboring the right-angled points are selected as vector points.
The method of Japanese application 64-68889 has the following problem. In the case of a projecting straight line having a width corresponding to one pixel, the center of a boundary pixel is used twice in defining an outline. Thus, the width of such a projecting straight line remains corresponding to one pixel even when an image is enlarged or expanded.